There has been an Israeli military operation in the Gaza Strip in the northern town of Beit Hanoun.
Palestinian medical sources say two Palestinian national security officers were killed and six people injured.
Palestinian security sources said armoured vehicles backed by helicopter gunships entered the town around 0300 local time (0100 GMT) on Monday.
There were clashes with local gunmen and the Israeli forces surrounded a police station.
An Israeli army spokesman said soldiers had returned fire when they were shot at from the station.
He said this was a search-and-arrest mission based on information received and that three men were picked up.
Israeli campaign 'eased'
Beit Hanoun is an area from which Palestinian militias launch primitive Qassam rockets at a nearby Israeli town, but none had been fired since last week.
Contrary to Palestinian expectations, Israel eased up its military campaign in Gaza when the war with Iraq started.
It had been targeting the Islamic militant group Hamas for weeks.
On Monday the Palestinian Health Ministry said 241 Palestinians had died since the beginning of the year in Israeli raids or because of military restrictions.
Many of them were civilians, and more than half of them were in Gaza.